Many work situations require workers to be positioned on top of platforms or vehicles that cannot be practically protected by a guardrail system enclosing the work area. To minimize the risk of a fall from such elevated positions and should there be a fall, to minimize serious or mortal injuries, various fall protection systems can be used. In general, fall arrest or travel restraint systems are designed to prevent the worker from reaching an unprotected edge or, in the event off a fall, to manage the distance and deceleration before the worker impacts a lower level. While an energy-absorbing device, is usually incorporated between a worker's safety harness and their anchorage system, many anchorage systems include some energy absorption.
Such systems typically include a roof anchorage or spaced anchorages including a horizontal lifeline extending between anchorages secured to a surface structure, such as a roof of a building; the safety harness worn by the worker; and a flexible tether line or “lanyard” interconnecting the anchorage or horizontal lifeline to the harness. The roof anchorage apparatus in the subject application is typically referred to in the fall arrest industry as a “tip over post” or a “force management anchor” (FMA).
In the event of a fall, the forces associated with the fall are generally parallel to the surface of the support structure and typically perpendicular to the FMA, extending generally parallel to the surface of the support structure and then over an edge thereof, or from a horizontal lifeline connected between two or more anchors, pressed into service due to the fall. Fall arrest loading includes a user directly connected to the FMA or connected to a horizontal lifeline (HLL) spanning between spaced FMAs. When subjected to a large force, such as when arresting a fall, the FMA typically rotates until the tip of the post nears the base plate and the forces are adjacent and nearly parallel to the base.
One useful purpose of a FMA is to absorb energy from a horizontal force while protecting the integrity of a generally weak roof envelope/membrane. The perpendicular force imparted into the post imparts a tipping moment into the post and likewise into the base. Fasteners, located on the base plate at an opposing side from whence the force is imposed, are placed to optimize a pull-out or tear-out resistance. Many surfaces, such as wood or sheet metal, have a limited and finite capacity to resist a moment or pull-out load imparted thereto but do have a much greater capacity to resist parallel shear forces in-line with the roof membrane.
Thus, typically a conventional FMA's repositions the leverage of the force from a maximum moment to a minimum moment adjacent the base plate so as to take advantage of the much greater strength along a plane of the roof inline more so with the base. However, Applicant has noted that FMA's appear to predominantly consider the initial moment exerted on the post, and thus upon the roof structure, at the point of release when the post leaves an orientation substantially perpendicular to the surface. The design load, post and base plate apparatus is such that the base plate and roof are capable of withstanding the initial loading at a maximal moment arm and maximal torque.
However, in some cases once a conventional FMA begins to tip, very little energy is absorbed as the FMA rotates towards the base, from a maximum moment to its minimum moment. Accordingly, the worker remains substantially in a period of free fall before the post reaches its minimum moment and least energy absorption, leaving additional fall energy that must be transferred to the remainder of the fall arresting system. Thus, and if the capacity of the system is exceeded, then the additional energy is transferred into the roof and the worker's body, which may lead to failure of the anchorage of the system and/or injuries to the worker.
Other FMA designs do include varying degrees of energy absorption, varying from negligible to some devices that deploy at a fairly constant force. In all cases, the total fall distance of the worker using FMAs is always greater than would occur if the anchorage was absolutely rigid.
Applicant believes that it is not physically possible to design an FMA that will reduce the total fall distance over that provided by an absolutely rigid anchorage. Absolutely rigid anchorages are frequently difficult to achieve without great expenditure and thus the sole purpose of such FMAs is to protect a weak anchorage.
For example, the SpiraTech™ “RoofSafe® Roof Anchor” available from Uniline Safety Systems Limited include a coiled tensile member encapsulated in a shell that breaks open once a tensile force is applied and deploys the tension member which unravels, thus initially absorbing some energy transferred to the hold-down fasteners on the roof from the falling worker.
In another example, the Miller “Fusion™ Roof Anchor Post” from Honeywell includes a built in energy absorbing component enclosed within a cylindrical shell. The energy absorbing component (tensile member) extends within the shell as the cylinder tips over when a horizontal force is applied, thus absorbing some of the energy.
The above examples absorb energy primarily down the axis of the tensile member or HLL because the initial force initiates the re-orientation of the force from large lever or moment arm when the FMA is perpendicular, to a small lever or short moment arm for the tensile force when it comes more in line with the post base. The energy associated with a falling worker can potentially injure the worker and potentially cause a failure in the connection between the FMA and the roofing membrane if they exceed the capacity of the fall arresting system.
Thus, it is Applicant's position that FMAs currently on the market have focused on protecting a generally weak anchorage or protecting a cladding layer of roof structure from the overturning torque or moment that may be applied by the FMA. The main intent to date has been to create an anchorage that stands above the roof surface to elevate the connection point of the user, but when a fall arrest loading is applied the purpose of the design has been to promptly lay the post down so that the forces are imparted into the roof structure primarily as a direct shear as close as possible to an outer cladding layer. The cladding layer has relatively low strength to resist a substantial overturning torque, but has considerable strength through membrane action to resist a shear applied along its surface. Therefore, a first consideration has been to reposition the forces closer to the anchorage. Conventional FMA's are mainly concerned with the torque that initiates tipping of the post and thereafter allow the post to rotate to a stronger position close to the base plate or roof surface, taking advantage of the much greater strength of putting the horizontal forces into the horizontal plane of the roof. However, many conventional FMA's provide little resistance after initial tip over and are substantially freely rotating or freely spooling throughout the rotation until the sudden arrest of the worker when the post parallels the roof line. Also, the conventional FMA's do not optimize the opportunity for energy absorption within the FMA itself.
Secondly, an important consideration in arresting the fall of a worker is that it is desirable for most the energy generated by the fall to be absorbed by the fall arresting system. When FMA deploys, as a function of absorbing energy it will actually allow the worker to fall somewhat further. However, when some quantity of energy is absorbed by the FMA, the worker will not accelerate as quickly or as much during the deployment of the FMA. Physics dictates that it is impossible to begin decelerating a worker connected to a horizontal lifeline at the instant that the horizontal lifeline begins to sag because the lifeline must first deflect until a tension in the worker's lanyard equals the weight of the worker (to counter the pull from gravity). Beyond this sag, known as the deceleration onset sag, the arresting force becomes greater than the worker's weight and the worker begins to slow down.
The remaining fall energy, at the stage where the FMAs have fully deployed, must be dissipated by other elements of the fall arrest system, such as additional stretch of the HLL, which will greatly increase the forces, but mostly by the deployment of a personal energy absorber that the worker has located between his harness and the HLL. This excess energy requires increased deployment of the personal energy absorber, and always increases the total fall distance of the worker and therefore increases the probability that the worker may strike the ground or a lower surface. There are instances with some of the existing FMA designs, where the increased energies gained by the worker due to their deployment of inefficient FMAs have exceeded the capacity of other energy absorbing mechanisms designed into the system, resulting in injurious impacts to the worker and damage to the roof the FMA is attached, to, possibly leading to complete failure of the anchorages.
Thus, the more energy that a FMA absorbs as it deploys, the sooner the fall energy of the worker is dissipated, the shorter the total fall distance of the worker, and the lower the probability of striking a lower surface and the lower the probability that larger impact forces may develop that may injure the worker or threaten the integrity of the anchorage of the system.
There is, therefore, a need in the art for an FMA having improved energy absorption when resisting a horizontal force while maintaining the integrity of a weak roof envelope/membrane, which only has a limited and finite capacity to resist pull out moments and a much greater capacity to resist horizontal forces once in line with the roof membrane.